Clinical examination of female 129/J mice from an in-house production colony revealed protrusions and tumor-like masses of the vulva in 47 of 106 (44%) animals. These neoplastic-like lesions were initially recognized in mice 5 to 9 months of age, but were subsequently noted in females as young as 6 weeks of age. Cultural, serological, histologic, and electron microscopic examinations were performed in an effort to characterize and determine the etiology of these lesions. Cultural and serological results were negative for the known murine bacterial, mycoplasmal, parasitic and viral pathogens. Microscopic examination revealed invasive carcinomas of the ventral vulva, characterized by multiple coalescing nests of epithelial cells extending downward from the surface. There proliferative lesions were very site-specific and characteristically located posterior to the vaginal opening between the orifices of the urethra and the clitoral gland duct. Invasive cells exhibited predominantly a well-differentiated squamous morphology forming solid nests, with scattered foci of proliferative epithelium with cystic or mucoid differentiation. Electron microscopy did not reveal any distinguishing diagnostic features nor evidence of viruses. Endometrial hyperplasia was invariably present in all tumor-bearing mice; interstitial cell hyperplasia of the ovary was seen less frequently. No other significant findings were observed in examination of other major organs. Assays of feed, water, bedding, and environmental air were negative for the presence of microbial pathogens and for detectable amounts of toxic chemicals including pesticide residues, formaldehyde and ammonia. Sporadic cases of vulvar squamous cell carcinoma have been reported in cattle, sheep, non-human primates, and humans, and have been associated with exposure to sunlight or papillomaviruses. Vulvar carcinomas are rarely seen in mice and have not been reported in the 129/J strain. The etiology of these tumors remains unknown, however, an estrogenic effect is being investigated.